Cold roll forming method for reducing a diameter of a metal pipe, and a metal pipe product having its diameter reduced by such method

ABSTRACT

A cold roll forming method for reducing a diameter of a metal pipe and a metal pipe product having a diameter reduced by such a method are disclosed, wherein the method includes: passing a raw metal pipe through a set of pre-forming rolls so that it can be pre-formed into an intermediate metal pipe having an elliptical, elongated circular or rectangular cross-sectional shape; and then passing the intermediate metal pipe through a set of diameter-reducing rolls disposed on a downstream side of the set of pre-forming rolls so that it can have its outer circumferential length reduced to be smaller than an outer peripheral length of the raw metal pipe, while at the same time being re-formed into a circular cross-sectional shape or any other cross-sectional shape different from the cross-sectional shape of the intermediate metal pipe.

This application is a continuation of U.S. application Ser. No.11/434,760, which is a continuation of U.S. application Ser. No.10/381,341, which is the National Stage of International Application No.PCT/JP01/08310, filed Sep. 25, 2001.

BACKGROUND

1. Technical Field

The present invention relates to a cold roll forming method for reducinga diameter of a metal pipe, and a metal pipe product having its diameterreduced by such method. More particularly, the present invention relatesto a metal pipe so formed that its diameter can be reduced by the coldroll forming process.

2. Prior Art

There are a number of conventional cold roll forming methods, morespecifically, cold diameter-reducing methods, whereby a diameter of ametal pipe may be reduced. These methods are briefly described below.

One method is provided for cold-reducing a steel pipe by using a 3-rollreducer (Japanese patent application as published under No. H7(1995)-51707). This method is specifically designed to eliminate anypossible irregular wall thickness that may occur on welded joints of thesteel pipe. To this end, the method includes a reducing step in which asteel pipe may be reduced by passing it through three or more reducerstands, and a sizing step in which a steel member resulting from thereducing step may be sized by passing it through a two-roll sizingdevice on two or more roll stands.

Another method uses a die that has a particular pattern formed around aninner circumferential surface thereof, through which a steel pipe may bepassed and drawn so that the steel pipe can have a pattern ofalternating grooves and ridges formed like a twill weave running along alength thereof (Japanese patent application as published under No. H7(1995)-314031). More specifically, a die that has a gear-like pattern onthe inner circumferential surface thereof is combined with a plugdesigned to reduce a wall thickness of a raw steel pipe. A finishedsteel pipe may thus be provided by applying a strong compressive forceagainst the wall thickness of the steel pipe, thereby causing the wallthickness to be deformed under this applied compressive force.

A further method is designed for cold roll forming anelectro-resistance-welded steel pipe. A final-stage roll stand that islocated on an extreme downstream side includes a set of rolls having aparticular roll profile, with a bore formed by the set of rolls beingrounded to conform to the roll profile (Japanese patent application aspublished under No. H6 (1996)-142718).

The above mentioned methods may serve their respective cold-working orcold-roll-forming purposes, but any of the methods has problems in thata machine that is used to implement this method is designed forparticular applications, there are restrictions on size (angle) of asteel pipe that can be cold-worked or rolled, a number of roll standsmust be increased, and the like.

SUMMARY OF THE INVENTION

It is therefore one object of the present invention to provide acold-working method, more specifically, a cold roll forming method, thatprovides an easy and reliable manner for cold roll forming raw metalpipes by using sets of diameter-reducing rolls, without having toincrease a number of roll stands used, wherein the method may be used toroll a raw metal pipe so that it can have a reduction in terms of itsdiameter, or to roll the raw metal pipe so that it can have a pattern ofalternating grooves and ridges on inner and/or outer circumferentialsurface(s) thereof while at the same time having reduction in terms ofits diameter, or to roll a double pipe including an outer pipe and aninner pipe so that it can have a reduction in terms of its diameter.

Another object of the present invention relates to a metal pipe productthat has its diameter reduced by using such method.

The present invention solves the problems associated with the prior artmethods described above, by providing a cold roll forming method thatincludes a pre-forming step of pre-forming a raw metal pipe into anintermediate metal pipe by passing it through a set of pre-formingrolls, followed by a diameter-reducing step of reducing a diameter ofthe intermediate metal pipe by passing it through a set ofdiameter-reducing rolls arranged in tandem with the set of pre-formingrolls.

More specifically, the cold roll forming method according to the presentinvention solves the above problems by allowing a raw metal pipe to bepassed through a set of pre-forming rolls so that it can be formed intoan intermediate metal pipe having any of an elliptic, elongatedcircular, rectangular and polygonal cross sectional shape, and thenallowing the intermediate metal pipe to be passed through a set ofdiameter-reducing rolls located downstream of the set of pre-formingrolls so that it can be re-formed into a final metal pipe having acircular cross section or any other cross-sectional shape that isdifferent from the cross-sectional shape of the intermediate metal pipe,thereby producing a final metal pipe having its outer circumferentiallength reduced to a smaller length than an outer circumferential lengthof the raw metal pipe.

It may be understood from the above description that in the cold rollforming method according to the present invention, a diameter-reducingprocess may be performed by passing a raw metal pipe through a set ofpre-forming rolls so that it can be pre-formed into an intermediatemetal pipe having a particular cross-sectional shape, and then passingthe intermediate metal pipe through a set of diameter-reducing rollsarranged in tandem with the set of pre-forming rolls so that it can beformed into a final metal pipe having a reduction in terms of itsdiameter. In this method, a pre-forming step may occur in a single pass,and a diameter-reducing step that follows the pre-forming step may alsooccur in a single pass. Alternatively, other methods are possible,depending upon a particular type of material on which a raw metal pipeis based, a particular wall thickness required for the raw metal pipe, aparticular diameter reduction rate required for the raw metal pipe,particular usage of a finished metal pipe, and other particularrequirements. Some of these other methods are described below.

For example, a process according to one possible method may includemultiple pre-forming steps that correspond to multiple sets ofpre-forming rolls, and multiple diameter-reducing steps that correspondto multiple sets of diameter-reducing rolls, wherein these sets ofpre-forming rolls and these sets of diameter-reducing rolls are arrangedin tandem such that each set of pre-forming rolls is followed by eachset of diameter-reducing rolls. This process allows for multiple passesof a raw metal pipe through each succeeding combination of one set ofpre-forming rolls and one set of diameter-reducing rolls.

A process according to another possible method may include a singlepre-forming step that corresponds to a single set of pre-forming rollsor multiple pre-forming steps that correspond to multiple sets ofpre-forming rolls arranged in tandem with each other, and a singlediameter-reducing step that corresponds to a single set ofdiameter-reducing rolls or multiple diameter-reducing steps thatcorrespond to multiple sets of diameter-reducing rolls arranged intandem with each other. This process allows for a single pass ormultiple passes of a raw metal pipe through the single or multiple setsof pre-forming rolls, and allows for a single pass or multiple passes ofa resulting metal pipe through the single or multiple sets ofdiameter-reducing rolls.

Any of the methods described above allows for use of a raw metal pipethat has any cross-sectional shape, such as circular, elliptical,elongated circular, rectangular, triangular, pentagonal, hexagonal, andother polygonal shapes.

According to the methods described above, a raw metal pipe that has anyof the above-mentioned cross-sectional shapes may be pre-formed into anydifferent cross-sectional shape other than an original cross-sectionalshape of the raw metal pipe. For example, when the raw metal pipe has acircular cross-sectional shape, it may be pre-formed into any of anelliptical, elongated circular, rectangular and polygonalcross-sectional shape. Similarly, when the raw metal pipe has any of anelliptical, elongated circular, rectangular and polygonalcross-sectional shape, it may be pre-formed into a circularcross-sectional shape. As another example, when the raw metal pipe has asquare or rectangular cross-sectional shape, it may be pre-formed into arectangular or square cross-sectional shape, respectively. As a furtherexample, when the raw metal pipe has an elliptical, elongated circular,rectangular or polygonal cross-sectional shape, it may be pre-formedinto an elliptical, elongated circular, rectangular or polygonalcross-sectional shape, in which case any of these cross-sectional shapesmay be provided by moving and rotating the raw metal pipe in itscircumferential direction. It should be noted, however, that when theraw metal pipe having the circular cross section is pre-formed into thesame cross-sectional shape, it is difficult to reduce a diameter of suchpre-formed metal pipe at a diameter-reduction rate of more than 3% whenit is passed through a subsequent set of diameter-reducing rolls. Insuch case, it is desirable that the raw metal pipe having the circularcross section should be pre-formed into any of an elliptical, elongatedcircular, rectangular, triangular, pentagonal, hexagonal or any otherpolygonal cross-sectional shape.

A raw metal pipe may be pre-formed into any of the cross-sectionalshapes shown above, by using an appropriate number of rolls in each setof pre-forming rolls, or by providing such rolls that have anappropriate cross-sectional shape, or by combining such multiple sets ofpre-forming rolls arranged in tandem. For example, when a raw metal pipeis to be pre-formed into a rectangular cross-sectional shape on one rollstand, the roll stand may include two such pre-forming rolls 2 a, 2 b asshown in FIG. 1(a), or four such pre-forming rolls 2 a, 2 b, 2 c, 2 d asshown in FIG. 1(b). Generally, when a raw metal pipe is pre-formed intoan elliptical or elongated circular cross-sectional shape on one rollstand, the roll stand may include two pre-forming rolls as shown in FIG.1(a), and when a raw metal pipe is pre-formed into any polygonalcross-sectional shape such as triangular on one roll stand, the rollstand may include two or three pre-forming rolls. Furthermore, when araw metal pipe is pre-formed into any polygonal cross-sectional shapesuch as pentagonal on one roll stand, the roll stand may include four orfive pre-forming rolls. In addition, when a raw metal pipe is pre-formedinto any polygonal cross-sectional shape such as hexagonal on one rollstand, the roll stand may include four or six pre-forming rolls.

In accordance with the cold roll forming methods of the presentinvention that have been described so far, a final metal pipe having itsouter circumferential length reduced to be smaller than an outercircumferential length of a raw metal pipe has any particularcross-sectional shape such as circular, elliptical, elongated circular,rectangular and polygonal cross-sectional shapes. This may beaccomplished by using an appropriate number of rolls in each set ofdiameter-reducing rolls, or by providing such rolls that have anappropriate cross-sectional shape, or combining such multiple sets ofdiameter-reducing rolls arranged in tandem.

A metal pipe that has been pre-formed into any particularcross-sectional shape through the set of pre-forming rolls may have itsdiameter reduced over its total length by passing this pre-formed metalpipe through the set of diameter-reducing rolls. Alternatively, somepart of a raw metal pipe may be pre-formed into any particularcross-sectional shape through the set of pre-forming rolls, and thendiameter-reducing rolling is performed by the diameter-reducing rolls onthis pre-formed part, whereby a final metal pipe in which some part ofit is reduced in its outer circumferential direction is obtained.

Each of the rolls in the set of diameter-reducing rolls may have aparticular pattern of grooves and ridges on a circumferential surfacethereof. By passing a metal pipe pre-formed by a preceding set ofpre-forming rolls through a following set of diameter-reducing rolls,the metal pipe may have a pattern of ridges and grooves formed on asurface while at the same time having its diameter reduced, when it ispassed through the set of diameter-reducing rolls having the beforedescribed particular pattern.

In this case, the cold roll forming methods of the present inventionallow a final metal pipe to have a pattern of ridges and grooves formedon an outer circumferential surface according to the pattern provided onthe diameter-reducing rolls by causing an outer circumferential surfaceof a pre-formed metal pipe to engage the surface of thediameter-reducing rolls, while allowing the final metal pipe to have itsdiameter reduced by the diameter-reducing rolls. One complete revolutionof the diameter-reducing rolls corresponds to one pitch of the patternbeing formed on the outer circumferential surface of the final metalpipe.

In the prior art cold roll forming method mentioned earlier wherein ametal pipe can have a pattern formed thereon by drawing it through adie, the metal pipe can only have the pattern formed in its longitudinaldirection. In contrast to the prior art method, the method according tothe present invention allows a metal pipe to have a pattern formed on asurface such that the pattern can be changed periodically for everypitch of the diameter-reducing rolls.

For example, when rolls in a set of diameter-reducing rolls have apattern of ridges and grooves on respective surfaces thereof that arerunning in a direction in which the rolls rotate, a final metal pipehaving its diameter reduced may have the same pattern of alternatinggrooves and ridges on a surface thereof that are running in alongitudinal direction of the metal pipe, or running in straight linesparallel with a central axis of the metal pipe, when it is passedthrough the set of diameter-reducing rolls. The pattern of alternatinggrooves and ridges on the diameter-reducing rolls may be modified todifferent patterns, such as a pattern of alternating grooves and ridgesthat appear as oblique lines inclined at an angle with regard to thecentral axis of the metal pipe, a pattern of alternating grooves andridges that appear as arc lines, a pattern of alternating grooves andridges that appear as spiral lines, and any other desired pattern ofalternating grooves and ridges.

In the cold roll forming methods of the present invention described sofar, a pattern molding die may be provided in a bore formed by a set ofdiameter-reducing rolls. This pattern molding die has a pattern ofridges and grooves on an outer circumferential surface thereof, and canbe inserted into a pre-formed metal pipe. Thus, when this metal pipepasses through the bore formed by the diameter-reducing rolls, it mayhave the same pattern of ridges and grooves, formed by the patternmolding die, on an inner circumferential surface of the metal pipe whileat the same time having its diameter reduced by the set ofdiameter-reducing rolls.

A raw metal pipe that will be pre-formed and then have its diameterreduced according to a method of the present invention has a givenlength (on the order of 0.5 m to 10 m, for example). The pattern moldingdie may be supported by any supporting member that may be provided toextend along a traveling course of the metal pipe from a downstream sidetoward an upstream side, or from the upstream side toward the downstreamside. In this way, the pattern molding die can be inserted into themetal pipe as the metal pipe passes through the bore formed by thediameter-reducing rolls. Thus, when the pattern molding die, that has aparticular pattern of ridges and grooves on the outer circumferentialsurface thereof, is provided in the bore formed by the diameter-reducingrolls, the metal pipe may have that particular pattern formed on aninner circumferential surface thereof while at the same time having itsdiameter reduced by the diameter-reducing rolls, as it passes throughthe diameter-reducing rolls.

For example, the metal pipe may have the pattern of ridges and groovesformed on the inner circumferential surface thereof such that the ridgesand grooves can appear as straight lines parallel with a central axis ofthe metal pipe, when the metal pipe passes through the set ofdiameter-reducing rolls. A supporting member that supports the patternmolding die may be provided so that it can be moved forward or backwardin a traveling direction of the metal pipe. In this way, the pattern canbe formed on any desired portion of the inner circumferential surface ofthe metal pipe while at the same the pipe has its diameter reduced bythe set of diameter-reducing rolls.

The pattern molding die having the particular pattern on its outersurface, coupled with the set of diameter-reducing rolls having theparticular pattern- on its outer surface, may provide respectivepatterns on inner and outer circumferential surfaces of a metal pipe asit passes through the set of diameter-reducing rolls.

The pattern molding die may be provided in the form of a roll, plug orthe like, and a particular pattern of ridges and grooves may be providedon its outer circumferential surface.

Now, an embodiment of the cold roll forming method according to thepresent invention that is specifically designed for use in working adouble pipe is described. In order to solve the problems mentioned abovein the “Prior Art” section, this cold roll forming method allows forworking of a double pipe including an inner pipe having a smallerdiameter and an outer metal pipe having a larger diameter, with theinner pipe being inserted into the outer metal pipe. Specifically, thecold roll forming method includes a pre-forming step that corresponds toa set of pre-forming rolls and a diameter-reducing step that correspondsto a set of diameter-reducing rolls, wherein this raw double pipe may bepassed through the set of pre-forming rolls so that its outer metal pipecan be pre-formed into any of an elliptical, elongated circular,rectangular and polygonal cross-sectional shape, and then thispre-formed double pipe may be passed through the set ofdiameter-reducing rolls so that its outer metal pipe can be re-formedinto a circular cross-sectional shape or any other cross-sectional shapedifferent from a pre-formed cross-sectional shape of the outer metalpipe, while at the same time at least a diameter of the outer metal pipeis reduced by the set of diameter-reducing rolls. In this double pipethus finished, the inner and outer pipes are united by causing part orall of an inner circumferential surface of the outer pipe to engage anouter circumferential surface of the inner pipe tightly.

It may be appreciated that this method provides an easy manner formanufacturing such double pipe products.

For example, a double pipe that may be cold-worked by this method mayinclude an inner pipe having a triangular cross-sectional shape and anouter pipe having a circular cross-sectional shape.

When such double pipe is passed through the set of diameter-reducingrolls, it may be re-formed such that an inner circumferential length ofthe outer pipe can become smaller than an outer circumferential lengthof the inner pipe, thereby causing an inner circumferential surface ofthe outer pipe to engage an outer circumferential surface of the innerpipe tightly. In this double pipe thus finished, the inner pipe andouter pipe may have respective outer circumferential surfaces and innercircumferential surfaces that engage each other tightly under applieduniform and equal pressure, and with a uniform thermal property. Thisfinal double pipe product thus obtained may provide highly improvedreliability.

In a process during which a double pipe is worked as described above,the double pipe is passed through one set of pre-forming rolls, and thenthrough one set of diameter-reducing rolls arranged in tandem with theset of pre-forming rolls. That is, a single pass occurs through the setof pre-forming rolls, and then a single pass occurs through the set ofdiameter-reducing rolls. Alternatively, other methods are possible,depending upon a particular type of material on which each of the innerand outer pipes is based, a particular wall thickness required for each,a particular diameter reduction rate required for each, a particularusage of a finished double pipe, and other particular requirements. Somethese other methods are described below.

For example, a process according to one possible method may includemultiple pre-forming steps that correspond to multiple sets ofpre-forming rolls, and multiple diameter-reducing steps that correspondto multiple sets of diameter-reducing rolls, wherein each combinationconsisting of each set of pre-forming rolls and each following set ofdiameter-reducing rolls is arranged in tandem with each followingcombination so that a double pipe can pass through each succeedingcombination. This process allows for multiple passes of the double pipethrough each succeeding combination of one set of pre-forming rolls andone set of diameter-reducing rolls.

A process according to another possible method may include a singlepre-forming step that corresponds to a single set of pre-forming rollsor multiple pre-forming steps that correspond to multiple sets ofpre-forming rolls arranged in tandem with each other, and a singlediameter-reducing step that corresponds to a single set ofdiameter-reducing rolls or multiple diameter-reducing steps thatcorrespond to multiple sets of diameter-reducing rolls arranged intandem with each other. This process allows for a single pass ormultiple passes of a double pipe through the single or multiple sets ofpre-forming rolls, and allows for a single pass or multiple passes of aresulting double pipe through the single or multiple sets ofdiameter-reducing rolls.

Like the methods described earlier in connection with a single metalpipe, any of the methods just described above allows for use of a rawdouble pipe including an inner pipe and an outer pipe, each of which hasany of cross-sectional shape, such as circular, elliptical, elongatedcircular, rectangular, triangular, pentagonal, hexagonal, and otherpolygonal shapes. Each of inner and outer pipes that have any of theabove-mentioned cross-sectional shapes may be pre-formed into anycross-sectional shape other than circular, elliptical, elongatedcircular, rectangular, triangular, pentagonal, hexagonal, and otherpolygonal shapes.

According to the cold roll forming methods that are used formanufacturing a double pipe, inner and outer pipes can be united withoutusing any bonding media. Thus, a double pipe thus obtained can providehighly improved reliability.

When a double pipe is to be formed according to any of the above coldroll forming methods, an inner pipe may be inserted into an outer pipeat any part or along a total length of the outer pipe.

When the inner pipe is inserted into the outer pipe along the totallength of the outer pipe, this double pipe that is passed through theset of pre-forming rolls and then through the set of diameter-reducingrolls includes inner and outer pipes that overlap each other over thetotal length thereof. When the inner pipe is inserted into the outerpipe at a desired part of the total length of the outer pipe, thisdouble pipe that is passed through the set of pre-forming rolls and thenthrough the set of diameter-reducing rolls includes inner and outerpipes that overlap each other at the desired part of the total length ofthe outer pipe. Thus, a wall thickness at an area of the double pipewhere the inner and outer pipes overlap each other can become greaterthan a remainder of the double pipe.

The inner pipe may be made of any metal. In this case, the metal may bethe same as, or different from, a metal usually used for the outer pipe.Alternatively, the inner pipe may be made of any non-metal material. Inthis case, the outer pipe may be made of steel, and the inner pipe maybe made of aluminum, titanium, or synthetic resin.

In any of the above cases, after a double pipe including inner and outerpipes is obtained by passing it through a set of pre-forming rolls andthen through a set of diameter-reducing rolls, this double pipe may beused as an inner pipe, and may be inserted into another metal pipe of agreater diameter. This triple pipe thus obtained may then be passedthrough the set of pre-forming rolls and then through the set ofdiameter-reducing rolls. Other multiple-layer pipes such as a quartetpipe, quintet pipe and the like may be obtained in the same manner.

When a double pipe is manufactured by using an appropriate cold rollforming method of the present invention described above, it may bereadily appreciated that the double pipe may be finished by passing itthrough a set of pre-forming rolls and then through a set ofdiameter-reducing rolls. Thus, what is required during a preliminarystage prior to a production stage is only to clean inner and outerpipes. In other words, the inner and outer pipes need not be subjectedto any thermal treatment or special machining prior to the productionstage.

For the same reasons, the rolls should not be made of special rollmaterials, but may be made of any roll material. The rolls need not besubjected to any surface treatment. Thus, the double pipe may befinished at less cost and more efficiently.

When a double pipe is pre-formed into any of an elliptical, elongatedcircular, rectangular, or polygonal cross-sectional shape during apre-forming process in the cold roll forming method, the pre-formingprocess may occur, with at least part of an inner circumferentialsurface of an outer metal pipe engaging an outer circumferential surfaceof an inner pipe. When a diameter-reducing process that follows thepre-forming process then occurs with the inner pipe being secured inposition, it may occur without causing the inner pipe to be misalignedwith regard to the outer pipe. When the diameter-reducing process thenoccurs with part of the inner pipe being inserted into the outer pipe,only portions of the inner and outer pipes overlapping each other canhave increased wall thickness, and this can be performed accurately.

Each of the diameter-reducing rolls that are used for the above purposesmay also have a particular pattern of ridges and grooves thereon. Then,a double pipe that has been pre-formed through the set of pre-formingrolls may have the particular pattern of ridges and grooves formedthereon while having its diameter reduced, as the double pipe is passedthrough the set of the diameter-reducing rolls. A double pipe productthat has thus been finished includes an outer metal pipe having theabove pattern formed on an outer circumferential surface thereof.

When a raw metal pipe is worked by using any of the cold roll formingmethods of the present invention, an arrangement including sets ofpre-forming rolls and sets of diameter-reducing rolls, or a raw metalpipe being worked, may be moved relative to the other in an axialdirection of the raw metal pipe.

When the raw metal pipe is to be moved relative to the arrangement ofthe pre-forming rolls and diameter-reducing rolls, this may beaccomplished by arranging the sets of pre-forming rolls and the sets ofdiameter-reducing rolls as follows.

For example, the arrangement may include several sets of pre-formingrolls and several sets of diameter-reducing rolls, wherein some or allof the rolls in each set of the pre-forming rolls and each set ofdiameter-reducing rolls may be linked operatively so that each of theserolls can be rotated. This allows a moving raw metal pipe to passthrough each set of pre-forming rolls and then through each setdiameter-reducing rolls.

One example of this arrangement is shown in FIG. 5(c), which is called aroll forming system (roll driving system). An example of the arrangementshown in FIG. 5(c) includes sets of pre-forming rolls 41 a, 41 b, 41 c,and sets of diameter-reducing rolls 42 a, 42 b, 42 c. As seen from FIG.5(c), the sets of pre-forming rolls and the sets of diameter-reducingrolls are arranged in tandem, and one set of pre-forming rolls isfollowed by one set of diameter-reducing rolls, which is followed byanother set of pre-forming rolls which is followed by another set ofdiameter-reducing rolls, and so on. In this arrangement, a raw metalpipe may be moved so that it can pass through each succeedingcombination of the sets of pre-forming rolls and diameter-reducing rollsfor a total of three times. When the sets of pre-forming rolls and thesets of diameter-reducing rolls are arranged in tandem, a particular setof pre-forming rolls or diameter-reducing rolls may be driven. Forexample, if one set includes four rolls, two rolls in the set may bedriven. In the example shown in FIG. 5(c), the set of diameter-reducingrolls 42 c includes four rolls, of which only two upper and lower rolls2 a, 2 c are driven, with remaining two side rolls 2 b, 2 d (not shown)being idler rolls.

In an alternative arrangement in which one set of pre-forming rolls andone set of diameter-reducing rolls include a plurality of rolls,respectively, and some or all of the rolls in respective sets ofpre-forming rolls and diameter-reducing rolls are idler rolls that arenot driven by any external drive source, a raw metal pipe may be passedthrough these sets of pre-forming rolls and diameter-reducing rolls inany of the following ways.

One way is to push a raw metal pipe forward into the set of pre-formingrolls, and into the set of diameter-reducing rolls by use of any pushingdevice. Specifically, the raw metal pipe may first be pushed into a boreformed by the set of pre-forming rolls located on an upstream side, andthen may be pushed into a bore formed by the set of diameter-reducingrolls located on a downstream side of the set of pre-forming rolls. Anexample of this method is shown in FIG. 5(a), which is called anextroll-forming system (which may also be called a non-driven roll pipepushing system). Examples of a pushing device that may be used for thispurpose include a hydraulically-operated cylinder, ahydraulically-operated jack and the like.

Another way is to draw a raw metal pipe out through the set ofpre-forming rolls, and through the set of diameter-reducing rolls towardthe downstream side by use of a drawing device. Specifically, the rawmetal pipe may first be drawn out through the bore formed by the set ofpre-forming rolls and then through the bore formed by the set ofdiameter-reducing rolls. One example of this system is shown in FIG.5(b), which is called a draw bench system (which may also be called anon-driven roll pipe drawing system). Examples of the drawing devicethat may be used for this purpose include a combination of a chuck thatis operated to catch a forward tip of a metal pipe and ahydraulically-operated jack that holds the chuck and pulls it bytraction force, or a combination of such chuck and a chain that isdriven to rotate circularly for moving the chuck by traction force.

Still another way is a combination of the two systems described above,wherein a raw metal pipe may be moved by the pushing device on theupstream side so that it can be pushed through the bore formed by theset of pre-forming rolls and through the bore formed by the set ofdiameter-reducing rolls, while the metal pipe is also pulled by thedrawing device on the downstream side so that it can be drawn outthrough the bore formed by the set of pre-forming rolls and through thebore formed by the set of diameter-reducing rolls.

Which of the three systems described above should be chosen may bedetermined, based on various factors, such as diameter, wall thickness,length, and rate of movement of a raw metal pipe being worked.

When any of these systems is chosen, the following considerations arerequired. For the extroll-forming system, the hydraulically-operatedcylinder tends to be lengthy, so it would be necessary to ensure thatthe cylinder is not longer. For the draw bench system, it would benecessary to ensure that no deformation occurs on an end of a metal pipebeing pulled. Finally, for the roll-forming system, it would benecessary to select an appropriate roll driving method.

In each of the arrangements shown in FIG. 5(a) and FIG. 5(b), one set ofpre-forming rolls 21 or 31 and one set of diameter-reducing rolls 22 or32 are arranged in tandem, wherein a raw metal pipe may be passedthrough the set of pre-forming rolls one time, and then may be passedthrough the set of diameter-reducing rolls one time.

Other arrangements are possible, although they are not shown. As oneexample, one set of pre-forming rolls 21 may be provided on an upstreamside, and several sets of diameter-reducing rolls 22 that are arrangedin tandem may be provided on a downstream side of the set of pre-formingrolls 21. As another example, several sets of pre-forming rolls 21 maybe arranged in tandem, and one set of diameter-reducing rolls 22 may beprovided on a downstream side of the sets of pre-forming rolls. As afurther example, several sets of pre-forming rolls 31 may be arranged intandem, and several sets of diameter-reducing rolls 32 that are arrangedin tandem may be provided on a downstream side of corresponding sets ofpre-forming rolls.

As a variation of the arrangement shown in FIG. 5(c), which is notshown, an arrangement may be such that it includes one set ofpre-forming rolls 21 a, followed by one set of diameter-reducing rolls42 a that is arranged in tandem with the set of pre-forming rolls. Inthis arrangement, one pass may occur through the set of pre-formingrolls, followed by one pass through the set of diameter-reducing rolls.As another variation, an arrangement may be such that it includes oneset of pre-forming rolls 41 a, followed by one set of diameter-reducingrolls 42 a, or followed by several sets of diameter-reducing rolls 42 a,42 b, 42 c arranged in tandem. As a further variation, an arrangementmay be such that it includes several sets of pre-forming rolls 41 a, 41b, 41 c arranged in tandem, followed by one set of diameter-reducingroll 42 a, or followed by several sets of diameter-reducing rolls 42 a,42 b, 42 c arranged in tandem.

In any of these variations, a metal pipe that has been pre-formed by aset of preceding pre-forming rolls may be passed through a following setof diameter-reducing rolls without causing the metal pipe to be pinchedout into a gap between rolls in the set of diameter-reducing rolls.

Any of these variations may also be applied to a double pipe, whereinthe double pipe that has been pre-formed by a preceding set ofpre-forming rolls may be passed through a following set ofdiameter-reducing rolls without causing an outer pipe to be pinched outinto a gap between rolls in the set of diameter-reducing rolls.

According to the various cold roll forming methods that have beendescribed so far, a diameter reduction rate of more than 3% may beattained each time that a metal pipe is passed through each succeedingcombination of a set of pre-forming rolls and set of diameter-reducingrolls. Experiments that were conducted by the inventors of the presentinvention show that a diameter reduction rate of at most 22% can beattained for each pass of a metal pipe through each combination of theset of pre-forming rolls and set of diameter-reducing rolls. Thediameter reduction rate may be varied in a range of between at least 3%and at most 22%, depending on respective types of pre-forming rolls anddiameter-reducing rolls that are used.

For example, when a raw metal pipe is made of iron, and has a wallthickness of 1.6 mm and an outer diameter of 48.6 mm, a diameterreduction rate of 22% may be attained by passing the raw metal pipethrough a pre-forming roll stand on which two pre-forming rolls aremounted so that it can be pre-formed into an intermediate metal pipehaving a triangular cross-sectional shape, and then by passing theintermediate metal pipe through a diameter reduction stand on whichthree diameter-reducing rolls are mounted so that it can have itsdiameter reduced by compressing the metal pipe in a direction ofcompression with its triangle vertexes being directed inwardly. Inanother example, a set of two pre-forming rolls is used for the samemetal pipe as in the previous example, but relative positions betweenthe two rolls are changed and a set of diameter-reducing rolls isreplaced by another set of diameter-reducing rolls. A diameter reductionrate that may be attained in this example is equal to 8%.

As a further example, when a raw metal pipe is made of iron, and has awall thickness of 4.5 mm and an outer diameter of 216.3 mm, a diameterreduction rate of 11.8% may be attained by passing the raw metal pipethrough a pre-forming roll stand on which two pre-forming rolls aremounted so that it can be pre-formed into an intermediate metal pipehaving a rectangular cross-sectional shape, and then by passing theintermediate metal pipe through a diameter reduction stand on which fourdiameter-reducing rolls are mounted so that it can have its diameterreduced by compressing the metal pipe in a direction of compression withits rectangle vertexes being directed inwardly. In another example, aset of two pre-forming rolls is used for the same metal pipe as in theprevious example, but relative positions between the two rolls arechanged and a set of diameter-reducing rolls is replaced by another setof diameter-reducing rolls. A diameter reduction rate that may beattained in this example is equal to 5%.

As still another example, when a raw metal pipe is made of iron, and hasa wall thickness of 4.5 mm and an outer diameter of 190.7 mm, a diameterreduction rate of 7.2% may be attained by passing the raw metal pipethrough a pre-forming roll stand on which four pre-forming rolls aremounted so that it can be pre-formed into an intermediate metal pipehaving a pentagonal cross-sectional shape, and then by passing theintermediate metal pipe through a diameter reduction stand on which fivediameter-reducing rolls are mounted so that it can have its diameterreduced by compressing the metal pipe in a direction of compression withits pentagon vertexes being directed inwardly. In another example, a setof four pre-forming rolls is used for the same metal pipe as in theprevious example, but relative positions between the four rolls arechanged and a set of diameter-reducing rolls is replaced by another setof diameter-reducing rolls. A diameter reduction rate that may beattained in this example is equal to 3%.

When a double pipe is passed through each succeeding combination of aset of pre-forming rolls and a following set of diameter-reducing rollsarranged in tandem with the set of pre-forming sets, a diameterreduction rate for an outer pipe that may be attained ranges between 3%and 22% for each pass through each combination.

Metal pipes obtained by the cold roll forming methods described so farhave been examined. Typically, when a raw metal pipe having a circularcross-sectional shape is pre-formed into an intermediate metal pipehaving a circular cross-sectional shape, and then the intermediate metalpipe has reduction in terms of its diameter, results show that a finalmetal pipe has an outer diameter precision that is equivalent to orbetter than that of the raw metal pipe. After cutting the final metalpipe to examine its outer diameter, it is found that a cutting plane hasan outer diameter that remains substantially the same as an originalouter diameter of the final metal pipe.

Various embodiments of the cold roll forming method according to thepresent invention have been described, and various metal pipes accordingto the present invention that may be obtained by using these methodshave also been described.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) illustrates one example of a set of pre-forming rolls that maybe used with the present invention, as viewed from a side thereof;

FIG. 1(b) illustrates another example of a set of pre-forming rolls thatmay be used with the present invention, as viewed from a side thereof;

FIG. 2(a) illustrates an example of a set of diameter-reducing rollsthat may be used with the present invention, as viewed in cross section;

FIG. 2(b) illustrates an example of a diameter reducing processincluding a set of pre-forming rolls, followed by a set ofdiameter-reducing rolls, as viewed from a side.

FIG. 3(a) is a perspective view illustrating a metal pipe of the presentinvention;

FIG. 3(b) is a perspective view illustrating another metal pipe of thepresent invention, with part of the metal pipe being broken away;

FIG. 4 illustrates an extroll-forming system, according to theembodiment of the present invention, that may be employed in conjunctionwith the method of the present invention, as viewed from a side;

FIG. 5(a) illustrates the extroll-forming system, according to theembodiment of the present invention, that may be employed in conjunctionwith the method of the present invention, as viewed in perspective;

FIG. 5(b) illustrates a draw bench system, according to the embodimentof the present invention, that may be employed in conjunction with themethod of the present invention, as viewed in perspective; and

FIG. 5(c) illustrates a roll-forming system, according to the embodimentof the present invention, that may be employed in conjunction with themethod of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Several preferred embodiments of the present invention will now bedescribed in further detail by referring to the accompanying drawings.It should be understood that arrangements, shapes and relative positionsof component elements or parts that are described below in connectionwith each of these embodiments are only shown by way of example, and aredescribed in general terms so as to enable any person skilled in the artto understand underlying concepts and principles of the presentinvention. It should also be understood that specific values andcompositions (materials) that are shown below for the component elementsor parts are given for illustrative purposes. It may be appreciated byany person skilled in the art that the present invention is not limitedto the embodiments that are provided in the following description, andtherefore may be varied in many ways without departing from the spiritand scope of the present invention as defined in the appended claims.

Embodiment 1

A raw pipe 1 that is subject to a diameter-reducing process is made ofsteel, and is originally formed into a round shape having an outerdiameter of 216.3 mm. As an initial step, round steel pipe 1 may bepassed through a set of pre-forming rolls 2 a and 2 b so that it can beformed into an intermediate steel pipe 1 a having a rectangular crosssection (FIG. 1(a)).

Then, the intermediate steel pipe 1 a may be passed through a set ofdiameter-reducing rolls 3 a, 3 b, 3 c and 3 d so that it can be formedinto a round steel pipe 1 b having a reduced diameter (FIGS. 2(a) and(b)).

During the diameter-reducing process, a hydraulically-operated cylinder4 may be operated so that its piston rod 5 can be moved forward in adirection of arrow 6 in FIG. 4, thereby moving the round steel pipe 1 inthat direction. There are cases in which a size, particularly a length,of the hydraulically-operated cylinder 4 should be reduced into aparticular size or length for some reasons. In those cases, a stroke Lof the piston rod 5 that can travel to its full length might becomeshorter than a length of a particular round steel pipe 1 beingprocessed. To avoid this, the piston rod 5 should preferably be providedwith an auxiliary rod that may be interposed between the round steelpipe 1 and the piston rod 5. In this way, the stroke L of the piston rod5 can be extended by a length of the auxiliary rod.

As shown in FIG. 2(a), each of the rolls 3 a, 3 b, 3 c, 3 d in the setof diameter-reducing rolls has a particular pattern of alternatinggrooves and ridges formed around an outer circumferential surfacethereof, running in a direction in which each roll rotates. When theintermediate steel pipe 1 a is then passed through the set ofdiameter-reducing rolls, an output, which is represented by the roundsteel pipe 1 b (FIG. 3(a)), may have a reduction in terms of itsdiameter, while at the same time it may have the same pattern of ridgesand grooves 1 c, formed around the outer circumferential surface of thediameter-reducing rolls, running along a length of the round steel pipe1 b.

Specifically, raw round steel pipe 1 has the outer diameter of 216.3 mmand a wall thickness of 8.2 mm. When it is passed through the set ofpre-forming rolls and then through the set of diameter-reducing rolls,round steel pipe 1 b that is thus finished may have a reduced diameterof 190.7 mm and a pattern of sixty (60) alternating grooves and ridges 1c formed to a depth of 0.7 mm on its outer circumferential surface,running along its length. In this case, a finished steel pipe had itsdiameter reduced at a diameter reduction rate of 11.8%.

The set of pre-forming rolls 2 a, 2 b shown in FIG. 1(a) may be replacedby another set of pre-forming rolls 2 a, 2 b, 2 c, 2 d shown in FIG.1(b).

In this case, a raw pipe is also made of steel, and has a round shape.It should be noted, however, that the raw pipe may also have any othercross sectional shape, and may also be made of any other material suchas stainless steel, aluminum and the like.

Embodiment 2

A raw pipe 7 is also made of steel, and has a round shape. Differentfrom the preceding embodiment, the raw pipe 7 is a double pipe includingan additional pipe 8 made of synthetic resin that is inserted into theraw steel pipe 7. A diameter reducing process includes a pre-formingstep, followed by a diameter reducing step, like in the precedingembodiment, but differs in that each of rolls 3 a, 3 b, 3 c, 3 d in aset of diameter-reducing rolls has no pattern of ridges and grooves onits circumferential surface.

By passing the raw steel pipe 7 through the pre-forming step and thenthrough the diameter reducing step, a double pipe 10 may be obtained,including the outer steel pipe 7 and the inner synthetic resin pipe 8 asshown in FIG. 3(b).

Specifically, when the raw steel pipe 7 is passed through the diameterreducing step, it may have a reduction in terms of its diameter. In thedouble pipe 10 thus obtained, the outer steel pipe 7 and the innersynthetic resin pipe 8 can be united by causing an inner circumferentialwall of the outer steel pipe 7 to engage an outer circumferential wallof the inner synthetic resin pipe 8 under uniform and equal pressure.

Embodiment 3

A steel pipe “STKM13A” (carbon steel pipe for machine structuralpurposes) as specified in relevant JIS specifications is used as a rawsteel pipe. The raw steel pipe has an outer diameter of 60.5 mm and awall thickness of 2.9 mm.

This embodiment uses a process shown in FIG. 5(b). A diameter reducingprocess may be performed by passing the raw steel pipe through a set oftwo pre-forming rolls so that it can be formed into an intermediatesteel pipe having an elliptical cross-sectional shape, and then bypassing the intermediate steel pipe through a set of twodiameter-reducing rolls so that it can be formed into a final steel pipehaving a reduction in terms of its diameter. The final steel pipe has anouter diameter of 58.2 mm, which corresponds to a diameter reductionrate of 3.8%.

Embodiment 4

A steel pipe “STKM13A” (carbon steel pipe for machine structuralpurposes) as specified in relevant JIS specifications is used as a rawsteel pipe. The raw steel pipe has an outer diameter of 63.5 mm and awall thickness of 1.2 mm.

This embodiment uses the process shown in FIG. 5(b). A diameter reducingprocess may be performed by passing the raw steel pipe through a set oftwo pre-forming rolls so that it can be formed into an intermediatesteel pipe having a rectangular cross-sectional shape, and then bypassing the intermediate steel pipe through a set of fourdiameter-reducing rolls so that it can be formed into a final steel pipehaving a reduction in terms of its diameter. The final steel pipe has anouter diameter of 60.0 mm, which corresponds to a diameter reductionrate of 5.5%.

Embodiment 5

A steel pipe “STKM13A” (carbon steel pipe for machine structuralpurposes) as specified in relevant JIS specifications is used as a rawsteel pipe. The raw steel pipe has an outer diameter of 63.5 mm and awall thickness of 2.3 mm.

This embodiment uses the process shown in FIG. 5(b). A diameter reducingprocess may be performed by passing the raw steel pipe through a set oftwo pre-forming rolls so that it can be formed into an intermediatesteel pipe having a rectangular cross-sectional shape, and then bypassing the intermediate steel pipe through a set of fourdiameter-reducing rolls so that it can be formed into a final steel pipehaving a reduction in terms of its diameter. The final steel pipe has anouter diameter of 58.2 mm, which corresponds to a diameter reductionrate of 8.3%.

Embodiment 6

An aluminum pipe “A5052TD” as specified in relevant JIS specificationsis used as a raw aluminum pipe. The raw aluminum pipe has an outerdiameter of 60.0 mm and a wall thickness of 3.0 mm.

This embodiment uses the process shown in FIG. 5(b). A diameter reducingprocess may be performed by passing the raw aluminum pipe through a setof four pre-forming rolls so that it can be formed into an intermediatealuminum pipe having a rectangular cross section, and then by passingthe intermediate aluminum pipe through a set of four diameter-reducingrolls so that it can be formed into a final aluminum pipe having areduction in terms of its diameter. The final aluminum pipe has an outerdiameter of 58.2 mm, which corresponds to a diameter reduction rate of3.0%.

Embodiment 7

A raw pipe is a double pipe including an outer steel pipe having anouter diameter of 63.5 mm and a wall thickness of 1.2 mm, and an innersteel pipe having an outer diameter of 60.0 mm and a wall thickness of1.2 mm, with the inner steel pipe being inserted into the outer steelpipe over a total length thereof. The outer and inner steel pipes bothuse a steel pipe “STK400-E-G” as specified in relevant JISspecifications.

This embodiment uses a process shown in FIG. 5(a). A diameter reducingprocess may be performed by passing the raw steel pipe through a set oftwo pre-forming rolls so that it can be formed into an intermediatesteel pipe having an elliptical cross-sectional shape, and then bypassing the intermediate steel pipe through a set of two diameterreducing rolls so that it can be formed into a final double-steel pipehaving a reduction in terms of its diameter. The final double-steel pipehas an outer diameter of 60.0 mm, which corresponds to a diameterreduction rate of 5.5%.

In the final double-steel pipe thus obtained, the outer and inner steelpipes may be united by causing an inner circumferential wall of theouter steel pipe to engage an outer circumferential wall of the innersteel pipe tightly.

Embodiment 8

A raw pipe includes an outer steel pipe having an outer diameter of 63.5mm and a wall thickness of 1.2 mm, and an inner aluminum pipe having anouter diameter of 60.0 mm and a wall thickness of 3.0 mm, with the inneraluminum pipe being inserted into the outer steel pipe over a totallength thereof. The outer steel pipe uses a steel pipe “STK400-E-G” asspecified in relevant JIS specifications, and the inner aluminum pipeuses an aluminum pipe “A5052TD” as specified in relevant JISspecifications.

This embodiment uses the process shown in FIG. 5(b). A diameter reducingprocess may be performed by passing the raw pipe through a set of twopre-forming rolls so that it can be formed into an intermediate pipehaving a rectangular cross-sectional shape, and then by passing theintermediate pipe through a set of four diameter reducing rolls so thatit can be formed into a final double pipe having a reduction in terms ofits diameter. The final double pipe has an outer diameter of 60.0 mm,which corresponds to a diameter reduction rate of 5.5%.

In the final double pipe thus obtained, the outer steel pipe and theinner aluminum pipe may be united by causing an inner circumferentialwall of the outer steel pipe to engage an outer circumferential wall ofthe inner aluminum pipe tightly.

Embodiment 9

A steel pipe “STKM13A” (carbon steel pipe for machine structuralpurposes) as specified in relevant JIS specifications is used as a rawsteel pipe. The raw steel pipe has an outer diameter of 63.5 mm and awall thickness of 1.2 mm.

This embodiment uses a process shown in FIG. 5(c). A diameter reducingprocess may be performed by using several sets of pre-forming rolls andseveral sets of diameter reducing rolls. Specifically, the raw steelpipe may first be passed through a first set of two pre-forming rolls sothat it can be formed so as to have an elliptical cross-sectional shape.This resulting steel pipe may then be passed through a first set of twodiameter reducing rolls so that it can have a reduction in terms of itsdiameter. This resulting steel pipe may then be passed through a secondset of two pre-forming rolls so that it can be formed so as to have anelliptical cross section. This resulting steel pipe may then be passedthrough a second set of four diameter reducing rolls so that it can havea further reduction in terms of its diameter. This resulting steel pipemay then be passed through a third set of two pre-forming rolls so thatit can be formed so as to have an elliptical cross-sectional shape. And,this resulting steel pipe may finally be passed through a third set offour diameter reducing rolls so that it can have a further reduction interms of its diameter. The final steel pipe has an outer diameter of 56mm, which corresponds to a

diameter reduction rate of 11.8% by utilizing the preceding three setsof the pair of pre-forming rolls and diameter reducing rolls.

Embodiment 10

A steel pipe “STKM13A” (carbon steel pipe for machine structuralpurposes) as specified in relevant JIS specifications is used as a rawsteel pipe. The raw steel pipe has an outer diameter of 63.5 mm and awall thickness of 1.6 mm.

This embodiment uses the process shown in FIG. 5(b). A diameter reducingprocess may be performed by passing the raw steel pipe through a set oftwo pre-forming rolls so that it can be formed into an intermediatesteel pipe having a triangular cross-sectional shape, and then bypassing the intermediate steel pipe through a set of three diameterreducing rolls so that it can have a reduction in terms of its diameter.A final steel pipe thus obtained has an outer diameter of 49 mm, whichcorresponds to a diameter reduction rate of 22%.

Embodiment 11

A steel pipe “STKM13A” (carbon steel pipe for machine structuralpurposes) as specified in relevant JIS specifications is used as a rawsteel pipe. The raw steel pipe has an outer diameter of 63.5 mm and awall thickness of 1.2 mm.

This embodiment uses the process shown in FIG. 5(b). A diameter reducingprocess may be performed by passing the raw steel pipe through a set offour pre-forming rolls so that it can be formed into an intermediatesteel pipe having a rectangular cross-sectional shape, and then bypassing the intermediate steel pipe through a set of four diameterreducing rolls so that it can have a reduction in terms of its diameter.A final steel pipe thus obtained has an outer diameter of 56 mm, whichcorresponds to a diameter reduction rate of 11.8%.

Embodiment 12

A steel pipe “STKM13A” (carbon steel pipe for machine structuralpurposes) as specified in relevant JIS specifications is used as a rawsteel pipe. The raw steel pipe has an outer diameter of 63.5 mm and awall thickness of 1.2 mm.

This embodiment uses the process shown in FIG. 5(b). A diameter reducingprocess may be performed by passing the raw steel pipe through a set offive pre-forming rolls so that it can be formed into an intermediatesteel pipe having a pentagonal cross-sectional shape, and then bypassing the intermediate steel pipe through a set of five diameterreducing rolls so that it can have a reduction in terms of its diameter.A final steel pipe thus obtained has an outer diameter of 58.9 mm, whichcorresponds to a diameter reduction rate of 7.2%.

Embodiment 13

A steel pipe “STK400-E-G” (carbon steel pipe for general purposes) asspecified in relevant JIS specifications is used as a raw steel pipe.The raw steel pipe has an outer diameter of 190.7 mm and a wallthickness of 4.5 mm.

This embodiment uses the process shown in FIG. 5(a). A diameter reducingprocess may be performed by passing the raw steel pipe through a set oftwo pre-forming rolls so that it can be formed into an intermediatesteel pipe having a rectangular cross-sectional shape, and then bypassing the intermediate steel pipe through a set of four diameterreducing rolls so that it can have a reduction in terms of its diameter.A final steel pipe thus obtained has an outer diameter of 180 mm, whichcorresponds to a diameter reduction rate of 5.0%.

Embodiment 14

A steel pipe “STK400-E-G” (carbon steel pipe for general purposes) asspecified in relevant JIS specifications is used as a raw steel pipe.The raw steel pipe has an outer diameter of 216.3 mm and a wallthickness of 10.3 mm.

This embodiment uses the process shown in FIG. 5(a). A diameter reducingprocess may be performed by passing the raw steel pipe through a set offour pre-forming rolls so that it can be formed into an intermediatesteel pipe having a rectangular cross-sectional shape, and then bypassing the intermediate steel pipe through a set of four diameterreducing rolls so that it can have a reduction in terms its diameter. Afinal steel pipe thus obtained has an outer diameter of 190.7 mm, whichcorresponds to a diameter reduction rate of 11.0%.

Embodiment 15

A steel pipe “STK400-E-G” (carbon steel pipe for general purposes) asspecified in relevant JIS specifications is used as a raw steel pipe.The raw steel pipe has an outer diameter of 216.3 mm and a wallthickness of 4.5 mm.

This embodiment uses the process shown in FIG. 5(a). A diameter reducingprocess may be performed by passing the raw steel pipe through a set oftwo pre-forming rolls so that it can be formed into an intermediatesteel pipe having a rectangular cross-sectional shape, and then bypassing the intermediate steel pipe through a set of four diameterreducing rolls, each roll having a pattern of ridges and grooves runningin a rotational direction thereof, so that the intermediate steel pipecan have a reduction in terms of its diameter and a pattern of groovesand ridges extending along a length thereof. A final steel pipe thusobtained has an outer diameter of 190.7 mm, which corresponds to adiameter reduction rate of 11.0%, and has a pattern of sixty (60) ridgesand grooves formed to a depth of 0.6 mm.

Embodiment 16

A steel pipe “STK400-E-G” (carbon steel pipe for general purposes) asspecified in relevant JIS specifications is used as a raw steel pipe.The raw steel pipe has an outer diameter of 190.7 mm and a wallthickness of 4.5 mm.

This embodiment uses the process shown in FIG. 5(a). A diameter reducingprocess may be performed by passing the raw steel pipe through a set offour pre-forming rolls so that it can be formed into an intermediatesteel pipe having a rectangular cross-sectional shape, and then bypassing the intermediate steel pipe through a set of four diameterreducing rolls, each roll having a pattern of ridges and grooves runningin a rotational direction thereof, so that the intermediate steel pipecan have a reduction in terms of its diameter and a pattern of ridgesand grooves extending along a length thereof. A final steel pipe thusobtained has an outer diameter of 180 mm, which corresponds to adiameter reduction rate of 5.6%, and has a pattern of sixty (60) ridgesand grooves formed to a depth of 0.7 mm.

CONCLUSION

In all of Embodiments 1 to 16 described above, a raw metal pipe has acircular cross-sectional shape, and a finished metal pipe also has acircular cross-sectional shape. Then, each of these finished metal pipesobtained according to these embodiments was compared with eachcorresponding raw metal pipe with regard to an outer diameterdimensional precision, and results show that the finished metal pipe hasouter diameter dimensional precision that is equivalent to or betterthan that of the raw metal pipe. Specifically, in Embodiments 1 to 12where a draw bench process or roll-forming process occurs, the finishedmetal pipe had an outer diameter dimensional precision that is within±0.1% of that of the raw metal pipe. In Embodiments 13 to 16 where anextroll-forming process occurs, the finished metal pipe had an outerdiameter dimensional precision that is within ±1.0% of that of the rawmetal pipe.

After cutting each of the finished metal pipes obtained in Embodiments 1to 16 to examine its outer diameter, it was found that its outerdiameter is substantially the same as that of the finished metal pipe.

The finished metal pipe obtained in any of these embodiments includessome particular areas where the wall thickness is increased during thediameter-reducing process and other areas where the wall thicknessremains to be the same as the original wall thickness. Particularly, thewall thickness in those particular areas may be increased as thediameter reduction rate is increased, when the raw metal pipe goesthrough a most basic arrangement including one set of pre-forming rolls,followed by one set of diameter-reducing rolls. The areas where the wallthickness is increased may be determined by a shape of the pre-formingrolls as well as a shape of the diameter-reducing rolls. Thus, dependingupon a particular shape of a final metal pipe that is to be formed, aparticular diameter reduction rate that is to be attained, and aparticular type of roll-forming that is to be employed, a set ofpre-forming rolls having an appropriate shape and the set ofdiameter-reducing rolls having the appropriate shape may be combined.This allows the final metal pipe to have a uniform wall thickness aroundan entire circumference thereof when it has its diameter reduced by thecold roll forming method of the present invention.

INDUSTRIAL APPLICABILITY

According to the various embodiments of the cold roll forming method ofthe present invention that have been described in detail, a raw metalpipe may be passed through a set of pre-forming rolls so that it can bepre-formed into an intermediate metal pipe having a particularcross-sectional shape, and the intermediate metal pipe may then bepassed through a set of diameter-reducing rolls so that it can bere-formed into a circular or any other cross-sectional shape differentfrom a particular shape of the intermediate metal pipe, while at thesame time it may have its outer circumferential length reduced to besmaller than an original outer circumferential length of the raw metalpipe.

It may be appreciated from the preceding description that the presentinvention provides an easy and reliable manner for producing a finishedmetal pipe having its diameter reduced, or a finished metal pipe havingits diameter reduced and having a particular pattern of ridges andgrooves formed on either or both of inner and outer circumferentialsurfaces, or a finished double pipe including inner and outer pipes andhaving its diameter reduced, without having to increase a number of rollstands used for these purposes.

The cold roll forming methods according to the present invention may beused to produce metal pipes whose wall is thick or partly thick, at lesscost and in an easier manner than any of conventional cold roll formingmethods.

In the conventional cold roll forming methods that are specificallydesigned to produce electro-resistance-welded steel pipes and seamlesssteel pipes, those steel pipes that may be produced by a device that isprovided for this purpose should have a particular outer diameter andwall thickness that have previously been determined. In order to produceother such steel pipes that have any other outer diameter and wallthickness than those previously determined, it would be required tomodify the existing device or rolls, or otherwise to add another deviceor rolls so that they can meet particular requirements. This wouldrequire an additional cost that leads to a high cost. In contrast, thepresent invention allows a metal pipe having any desired outer diameterand wall thickness to be produced easily and at low cost, since itrequires no additional device or equipment.

When a metal pipe that requires higher strength on any particular partthereof is cold-rolled by a conventional method, the metal pipe istypically cold-rolled by increasing a wall thickness or strength notonly on that particular part but also along its total length. Incontrast, the present invention allows such metal pipe to be cold-rolledby only using a double pipe on that part requiring higher strength tosupplement insufficient strength threat. This can be achieved at lesscost since there is no need of increasing the wall thickness or strengthover the total length of the metal pipe, as is the case with theconventional methods.

In this regard, one advantage of the cold roll forming method accordingto the present invention is that a part of a metal pipe that requireshigher strength may be supplemented by a double pipe in which inner andouter pipes may be made to overlap each other on that part by simplyaligning the inner pipe with the outer pipe accurately.

In the conventional methods, a part of a metal pipe that requiressufficient strength may be supplemented by using a tailored blank,including a pipe having the same diameter and a different wallthickness, welded to the metal pipe. A further advantage of the presentinvention is that it allows a double pipe only to be provided on thatpart of a metal pipe that requires increased wall thickness, withouthaving to use such tailored blank. Thus, this can be achieved at lesscost.

When a metal pipe includes portions where a decreased wall thickness maybe produced during bending, bulge forming, or hydro-forming of the metalpipe, the present invention allows a double pipe only to be provided onthose portions to increase the wall thickness threat. This can beachieved at less cost and in an easier manner.

According to the present invention, a metal pipe, such as a single pipeand a double pipe, may initially be pre-formed into an intermediate pipehaving its diameter reduced, and the intermediate pipe may then bere-formed into a circular, triangular or any other cross-sectionalshape. Thus, the present invention provides an easy, reliable and lesscostly manner for producing multiple-layer steel pipe products havingany cross-sectional shape, as compared with prior art methods.

1. A cold roll forming method for reducing a diameter of a raw metalpipe, comprising: passing a raw metal pipe, having an originalcross-section, through a set of pre-forming rolls, thereby pre-formingsaid raw metal pipe into an intermediate metal pipe having one of anelliptical cross-section, elongated circular cross-section, rectangularcross-section, and polygonal cross-section, with the proviso that thecross-section of said intermediate metal pipe is different from theoriginal cross-section in terms of shape; and passing said intermediatemetal pipe through a set of diameter-reducing rolls positioneddownstream said set pre-forming rolls, thereby forming said intermediatemetal pipe into a pipe member having (i) an outer circumferential lengththat is smaller than an outer circumferential length of said raw metalpipe, and (ii) a cross-section that is different from the cross-sectionof said intermediate metal pipe in terms of shape.
 2. The cold rollforming method according to claim 1, wherein passing a raw metal pipethrough a set of pre-forming rolls comprises passing said raw metal pipethrough plural sets of pre-forming rolls, and passing said intermediatemetal pipe through a set of diameter-reducing rolls comprises passingsaid intermediate metal pipe through plural sets of diameter-reducingrolls, with at least some of said pre-forming rolls in each of saidplural sets of pre-forming rolls and at least some of saiddiameter-reducing rolls in each of said plural sets of diameter-reducingrolls being rotatably linked with one another, thereby allowing said rawmetal pipe to be pulled through said each of said plural sets ofpre-forming rolls and allowing said intermediate metal pipe to be pulledthrough said each of said plural sets of diameter-reducing rolls.
 3. Thecold roll forming method according to claim 1, wherein passing a rawmetal pipe through a set of pre-forming rolls comprises passing said rawmetal pipe through plural sets of pre-forming rolls, and passing saidintermediate metal pipe through a set of diameter-reducing rollscomprises passing said intermediate metal pipe through plural sets ofdiameter-reducing rolls, with at least some of said pre-forming rolls ineach of said plural sets of pre-forming rolls and at least some of saiddiameter-reducing rolls in each of said plural sets of diameter-reducingrolls being idler rolls that are free from an external driving force,thereby allowing said raw metal pipe to be moved through said each ofsaid plural sets of pre-forming rolls and allowing said intermediatemetal pipe to be moved through said each of said plural sets ofdiameter-reducing rolls by one of (i) pushing said raw metal pipe froman upstream side into a bore formed by said pre-forming rolls in saideach of said plural sets of pre-forming rolls, and thereby pushing saidintermediate metal pipe into a bore formed by said diameter-reducingrolls in said each of said plural sets of diameter-reducing rolls, (ii)drawing said raw metal pipe toward a downstream side into a bore formedby said pre-forming rolls in said each of said plural sets ofpre-forming rolls, and thereby drawing said intermediate metal pipe intoa bore formed by said diameter-reducing rolls in said each of saidplural sets of diameter-reducing rolls, and (iii) pushing said raw metalpipe from an upstream side into a bore formed by said pre-forming rollsin said each of said plural sets of pre-forming rolls and therebypushing said intermediate metal pipe into a bore formed by saiddiameter-reducing rolls in said each of said plural sets ofdiameter-reducing rolls, while drawing said raw metal pipe toward adownstream side into said bore formed by said pre-forming rolls in saideach of said plural sets of pre-forming rolls and thereby drawing saidintermediate metal pipe into said bore formed by said diameter-reducingrolls in said each of said plural sets of diameter-reducing rolls. 4.The cold roll forming method according to claim 1, wherein each of saiddiameter-reducing rolls of said set of diameter-reducing rolls has on asurface thereof a pattern of ridges and grooves, such that passing saidintermediate metal pipe through said set of diameter-reducing rollsresults in said pipe member having the pattern of ridges and groovesformed on a surface thereof.
 5. The cold roll forming method accordingto claim 1, wherein disposed in a bore formed by said diameter-reducingrolls, of said set of diameter-reducing rolls, is a molding die havingon an outer circumferential surface thereof a pattern of ridges andgrooves, with said molding die being inserted into said intermediatemetal pipe upon passing said intermediate metal pipe through said set ofdiameter-reducing rolls such that said pipe member has on an innercircumferential surface thereof the pattern of ridges and grooves. 6.The cold roll forming method according to claim 1, wherein passing saidintermediate metal pipe through said set of diameter-reducing rollsresults in a diameter of said intermediate metal pipe being reduced byat least 3%.
 7. A metal pipe product produced by the method of claim 1.8. A cold roll forming method for reducing a diameter of a raw metalpipe, comprising: passing a raw metal pipe, having an originalcross-section, through a set of pre-forming rolls, thereby pre-formingsaid raw metal pipe into an intermediate metal pipe having one of anelliptical cross-section, elongated circular cross-section, rectangularcross-section, and polygonal cross-section, with the proviso that thecross-section of said intermediate metal pipe is different from theoriginal cross-section in terms of shape; and passing said intermediatemetal pipe through a set of diameter-reducing rolls positioneddownstream said set pre-forming rolls, thereby forming said intermediatemetal pipe into a pipe member having (i) an outer circumferential lengththat is smaller than an outer circumferential length of said raw metalpipe, and (ii) a cross-section that is different from the cross-sectionof said intermediate metal pipe in terms of shape; passing said pipemember through another set of pre-forming rolls; and then passing saidpipe member through another set of diameter-reducing rolls.
 9. The coldroll forming method according to claim 8, wherein at least some of saidpre-forming rolls in said set of pre-forming rolls and said another setof pre-forming rolls, and at least some of said diameter-reducing rollsin said set of diameter-reducing rolls and said another set ofdiameter-reducing rolls, are rotatably linked with one another, therebyallowing said raw metal pipe to be pulled through said set ofpre-forming rolls, allowing said intermediate metal pipe to be pulledthrough said set of diameter-reducing rolls, and allowing said pipemember to be pulled through said another set of pre-forming rolls andsaid another set of diameter-reducing rolls.
 10. The cold roll formingmethod according to claim 8, wherein at least some of said pre-formingrolls in said set of pre-forming rolls and said another set ofpre-forming rolls, and at least some of said diameter-reducing rolls insaid set of diameter-reducing rolls and said another set ofdiameter-reducing rolls, being idler rolls that are free from anexternal driving force, thereby allowing said raw metal pipe to be movedthrough said set of pre-forming rolls, allowing said intermediate metalpipe to be moved through said set of diameter-reducing rolls, andallowing said pipe member to be moved through said another set ofpre-forming rolls and said another set of diameter-reducing rolls by oneof (i) pushing said raw metal pipe from an upstream side into a boreformed by said pre-forming rolls in said set of pre-forming rolls suchthat said intermediate metal pipe is pushed into a bore formed by saiddiameter-reducing rolls in said set of diameter-reducing rolls, and suchthat said pipe member is pushed into a bore formed by said pre-formingrolls in said another set of pre-forming rolls and into a bore formed bysaid diameter-reducing rolls in said another set of diameter-reducingrolls, (ii) drawing said raw metal pipe from a downstream side into abore formed by said pre-forming rolls in said set of pre-forming rollssuch that said intermediate metal pipe is drawn into a bore formed bysaid diameter-reducing rolls in said set of diameter-reducing rolls, andsuch that said pipe member is drawn into a bore formed by saidpre-forming rolls in said another set of pre-forming rolls and into abore formed by said diameter-reducing rolls in said another set ofdiameter-reducing rolls, and (iii) pushing said raw metal pipe from anupstream side into a bore formed by said pre-forming rolls in said setof pre-forming rolls such that said intermediate metal pipe is pushedinto a bore formed by said diameter-reducing rolls in said set ofdiameter-reducing rolls, and such that said pipe member is pushed into abore formed by said pre-forming rolls in said another set of pre-formingrolls and into a bore formed by said diameter-reducing rolls in saidanother set of diameter-reducing rolls, while drawing said raw metalpipe from a downstream side into said bore formed by said pre-formingrolls in said set of pre-forming rolls such that said intermediate metalpipe is drawn into said bore formed by said diameter-reducing rolls insaid set of diameter-reducing rolls, and such that said pipe member isdrawn into said bore formed by said pre-forming rolls in said anotherset of pre-forming rolls and into said bore formed by saiddiameter-reducing rolls in said another set of diameter-reducing rolls.11. The cold roll forming method according to claim 8, wherein each ofsaid diameter-reducing rolls, of at least one of said set ofdiameter-reducing rolls and said another set of diameter-reducing rolls,has on a surface thereof a pattern of ridges and grooves, such thatpassing said intermediate metal pipe through said set of diameterreducing rolls or passing said pipe member through said another set ofdiameter-reducing rolls results in said pipe member having the patternof ridges and grooves formed on a surface thereof.
 12. The cold rollforming method according to claim 8, wherein disposed in a bore formedby said diameter-reducing rolls, of at least one of said set ofdiameter-reducing rolls and said another set of diameter-reducing rolls,is a molding die having on an outer circumferential surface thereof apattern of ridges and grooves, with said molding die being inserted intosaid intermediate metal pipe or said pipe member upon passing saidintermediate metal pipe through said set of diameter-reducing rolls orpassing said pipe member through said another set of diameter-reducingrolls, such that said pipe member has on an inner circumferentialsurface thereof the pattern of ridges and grooves.
 13. The cold rollforming method according to claim 8, wherein passing said intermediatemetal pipe through said set of diameter-reducing rolls results in adiameter of said intermediate metal pipe being reduced by at least 3%,and passing said pipe member through said another set ofdiameter-reducing rolls results in a diameter of said pipe member beingreduced by at least 3%.
 14. A metal pipe product produced by the methodof claim
 8. 15. A cold roll forming method for reducing a diameter of araw metal pipe, comprising: passing a double pipe, including an originalouter metal pipe surrounding an inner metal pipe and having an originaldiameter, through a set of pre-forming rolls, thereby pre-forming saidouter metal pipe into an intermediate outer metal pipe having one of anelliptical cross-section, elongated circular cross-section, rectangularcross-section, and polygonal cross-section, with the proviso that thecross-section of said intermediate outer metal pipe is different fromthe original cross-section in terms of shape; and then passing saiddouble pipe through a set of diameter-reducing rolls positioneddownstream said set of pre-forming rolls, thereby forming saidintermediate outer metal pipe into an outer pipe member having (i) anouter circumferential length that is smaller than an outercircumferential length of said original outer metal pipe, and (ii) across-section that is different from the cross-section of saidintermediate outer metal pipe in terms of shape, with an innercircumferential surface of said outer pipe member being tightly engagedwith an outer circumferential surface of said inner metal pipe.
 16. Thecold roll forming method according to claim 15, further comprising:forming said double pipe by inserting said inner metal pipe into saidoriginal outer metal pipe such that said inner metal pipe and saidoriginal outer metal pipe overlap one another along a total length ofsaid original outer metal pipe.
 17. The cold roll forming methodaccording to claim 15, wherein passing a double pipe through a set ofpre-forming rolls comprises passing said double pipe through plural setsof pre-forming rolls, and passing said double pipe through a set ofdiameter-reducing rolls comprises passing said double pipe throughplural sets of diameter-reducing rolls, with at least some of saidpre-forming rolls in each of said plural sets of pre-forming rolls andat least some of said diameter-reducing rolls in each of said pluralsets of diameter-reducing rolls being rotatably linked with one another,thereby allowing said double pipe to be pulled through said each of saidplural sets of pre-forming rolls and allowing said double pipe to bepulled through said each of said plural sets of diameter-reducing rolls.18. The cold roll forming method according to claim 15, wherein passinga double pipe through a set of pre-forming rolls comprises passing saiddouble pipe through plural sets of pre-forming rolls, and passing saiddouble pipe through a set of diameter-reducing rolls comprises passingsaid double pipe through plural sets of diameter-reducing rolls, with atleast some of said pre-forming rolls in each of said plural sets ofpre-forming rolls and at least some of said diameter-reducing rolls ineach of said plural sets of diameter-reducing rolls being idler rollsthat are free from an external driving force, thereby allowing saiddouble pipe to be moved through said each of said plural sets ofpre-forming rolls and allowing said double pipe to be moved through saideach of said plural sets of diameter-reducing rolls by one of (i)pushing said double pipe from an upstream side into a bore formed bysaid pre-forming rolls in said each of said plural sets and into a boreformed by said diameter-reducing rolls in said each of said plural setsof diameter-reducing rolls, (ii) drawing said double pipe toward adownstream side into a bore formed by said pre-forming rolls in saideach of said plural sets of pre-forming rolls and into a bore formed bysaid diameter-reducing rolls in said each of said plural sets ofdiameter-reducing rolls, and (iii) pushing said double pipe from anupstream side into a bore formed by said pre-forming rolls in said eachof said plural sets of pre-forming rolls and into a bore formed by saiddiameter-reducing rolls in said each of said plural sets ofdiameter-reducing rolls, while drawing said double pipe toward adownstream side into said bore formed by said pre-forming rolls in saideach of said plural sets of pre-forming rolls and into said bore formedby said diameter-reducing rolls in said each of said plural sets ofdiameter-reducing rolls.
 19. The cold roll forming method according toclaim 15, wherein each of said diameter-reducing rolls of said set ofdiameter-reducing rolls has on a surface thereof a pattern of ridges andgrooves, such that passing said double pipe through said set ofdiameter-reducing rolls results in said outer pipe member having thepattern of ridges and grooves formed on a surface thereof.
 20. The coldroll forming method according to claim 15, wherein passing said doublepipe through said set of diameter-reducing rolls results in a diameterof said intermediate outer metal pipe being reduced by at least 3%. 21.A metal pipe product produced by the method of claim
 15. 22. A cold rollforming method for reducing a diameter of a raw metal pipe, comprising:passing a double pipe, including an original outer metal pipesurrounding an inner metal pipe and having an original diameter, througha set of pre-forming rolls, thereby pre-forming said outer metal pipeinto an intermediate outer metal pipe having one of an ellipticalcross-section, elongated circular cross-section, rectangularcross-section, and polygonal cross-section, with the proviso that thecross-section of said intermediate outer metal pipe is different fromthe original cross-section in terms of shape; then passing said doublepipe through a set of diameter-reducing rolls positioned downstream saidset pre-forming rolls, thereby forming said intermediate outer metalpipe into an outer pipe member having (i) an outer circumferentiallength that is smaller than an outer circumferential length of saidoriginal outer metal pipe, and (ii) a cross-section that is differentfrom the cross-section of said intermediate outer metal pipe in terms ofshape; then passing said double pipe through another set of pre-formingrolls; and then passing said double pipe through another set ofdiameter-reducing rolls, whereby an inner circumferential surface ofsaid outer pipe member is tightly engaged with an outer circumferentialsurface of said inner metal pipe.
 23. The cold roll forming methodaccording to claim 22, further comprising: forming said double pipe byinserting said inner metal pipe into said original outer metal pipe suchthat said inner metal pipe and said original outer metal pipe overlapone another along a total length of said original outer metal pipe. 24.The cold roll forming method according to claim 22, wherein at leastsome of said pre-forming rolls in said set of pre-forming rolls and saidanother set of pre-forming rolls, and at least some of saiddiameter-reducing rolls in said set of diameter-reducing rolls and saidanother set of diameter-reducing rolls, are rotatably linked with oneanother, thereby allowing said double pipe to be pulled through said setof pre-forming rolls, allowing said double pipe to be pulled throughsaid set of diameter-reducing rolls, and allowing said double pipe to bepulled through said another set of pre-forming rolls and said anotherset of diameter-reducing rolls.
 25. The cold roll forming methodaccording to claim 22, wherein at least some of said pre-forming rollsin said set of pre-forming rolls and said another set of pre-formingrolls, and at least some of said diameter-reducing rolls in said set ofdiameter-reducing rolls and said another set of diameter-reducing rolls,being idler rolls that are free from an external driving force, therebyallowing said double pipe to be moved through said set of pre-formingrolls, allowing said double pipe to be moved through said set ofdiameter-reducing rolls, and allowing said double pipe to be movedthrough said another set of pre-forming rolls and said another set ofdiameter-reducing rolls by one of (i) pushing said double pipe from anupstream side into a bore formed by said pre-forming rolls in said setof pre-forming rolls, into a bore formed by said diameter-reducing rollsin said set of diameter-reducing rolls, into a bore formed by saidpre-forming rolls in said another set of pre-forming rolls and into abore formed by said diameter-reducing rolls in said another set ofdiameter-reducing rolls, (ii) drawing said double pipe from a downstreamside into a bore formed by said pre-forming rolls in said set ofpre-forming rolls, into a bore formed by said diameter-reducing rolls insaid set of diameter-reducing rolls, into a bore formed by saidpre-forming rolls in said another set of pre-forming rolls and into abore formed by said diameter-reducing rolls in said another set ofdiameter-reducing rolls, and (iii) pushing said double pipe from anupstream side into a bore formed by said pre-forming rolls in said setof pre-forming rolls, into a bore formed by said diameter-reducing rollsin said set of diameter-reducing rolls, into a bore formed by saidpre-forming rolls in said another set of pre-forming rolls and into abore formed by said diameter-reducing rolls in said another set ofdiameter-reducing rolls, while drawing said double pipe from adownstream side into said bore formed by said pre-forming rolls in saidset of pre-forming rolls, into said bore formed by saiddiameter-reducing rolls in said set of diameter-reducing rolls, intosaid bore formed by said pre-forming rolls in said another set ofpre-forming rolls and into said bore formed by said diameter-reducingrolls in said another set of diameter-reducing rolls.
 26. The cold rollforming method according to claim 22, wherein each of saiddiameter-reducing rolls, of at least one of said set ofdiameter-reducing rolls and said another set of diameter-reducing rolls,has on a surface thereof a pattern of ridges and grooves, such thatpassing said double pipe through said set of diameter reducing rolls orpassing said double pipe through said another set of diameter-reducingrolls results in said outer pipe member having the pattern of ridges andgrooves formed on a surface thereof.
 27. The cold roll forming methodaccording to claim 22, wherein passing said double pipe through said setof diameter-reducing rolls results in a diameter of said outer metalpipe being reduced by at least 3%, and passing said double pipe throughsaid another set of diameter-reducing rolls results in a diameter ofsaid outer pipe member being reduced by at least 3%.
 28. A metal pipeproduct produced by the method of claim
 22. 29. The cold roll formingmethod according to claim 1, wherein said raw material pipe has a wallthickness within a range of from 1.6 mm to 10.3 mm.
 30. The cold rollforming method according to claim 8, wherein said raw material pipe hasa wall thickness within a range of from 1.6 mm to 10.3 mm.
 31. The coldroll forming method according to claim 15, wherein said double pipe hasa double wall thickness equal to a wall thickness of said original outermetal pipe plus a wall thickness of said inner metal pipe, with saiddouble wall thickness being within a range of from 1.6 mm to 10.3 mm.32. The cold roll forming method according to claim 22, wherein saiddouble pipe has a double wall thickness equal to a wall thickness ofsaid original outer metal pipe plus a wall thickness of said inner metalpipe, with said double wall thickness being within a range of from 1.6mm to 10.3 mm.
 33. The cold roll forming method according to claim 1,wherein said raw metal pipe has a wall thickness and a diameter, with aratio of said diameter to said wall thickness being within a range offrom 20.0 to 52.9, and passing said intermediate metal pipe through saidset of diameter-reducing rolls results in a diameter of saidintermediate metal pipe being reduced by 3%-22%.
 34. The cold rollforming method according to claim 8, wherein said raw metal pipe has awall thickness and a diameter, with a ratio of said diameter to saidwall thickness being within a range of from 20.0 to 52.9, passing saidintermediate metal pipe through said set of diameter-reducing rollsresults in a diameter of said intermediate metal pipe being reduced by3%-22%, and passing said pipe member through said another set ofdiameter-reducing rolls results in a diameter of said pipe member beingreduced by 3%-22%, with the proviso that total diameter reduction doesnot exceed 22%.
 35. The cold roll forming method according to claim 15,wherein said double pipe has a double wall thickness equal to a wallthickness of said original outer metal pipe plus a wall thickness ofsaid inner metal pipe, and said original outer metal pipe has adiameter, with a ratio of said diameter to said double wall thicknessbeing within a range of from 20.0 to 52.9, and passing said double pipethrough said set of diameter-reducing rolls results in a diameter ofsaid intermediate outer metal pipe being reduced by 3%-22%.
 36. The coldroll forming method according to claim 22, wherein said double pipe hasa double wall thickness equal to a wall thickness of said original outermetal pipe plus a wall thickness of said inner metal pipe, and saidoriginal outer metal pipe has a diameter, with a ratio of said diameterto said double wall thickness being within a range of from 20.0 to 52.9,passing said double pipe through said set of diameter-reducing rollsresults in a diameter of said intermediate outer metal pipe beingreduced by 3%-22%, and passing said double pipe through said another setof diameter-reducing rolls results in a diameter of said outer pipemember being reduced by 3%.-22%, with the proviso that total diameterreduction does not exceed 22%.
 37. A cold roll forming method forreducing a diameter of a raw metal pipe, comprising: passing a raw metalpipe, having an original cross-section, through plural sets ofpre-forming rolls, thereby pre-forming said raw metal pipe into anintermediate metal pipe having one of an elliptical cross-section,elongated circular cross-section, rectangular cross-section, andpolygonal cross-section, with the proviso that the cross-section of saidintermediate metal pipe is different from the original cross-section interms of shape; and passing said intermediate metal pipe through pluralsets of diameter-reducing rolls positioned downstream said plural setsof pre-forming rolls, thereby forming said intermediate metal pipe intoa pipe member having (i) an outer circumferential length that is smallerthan an outer circumferential length of said raw metal pipe, and (ii) across-section that is different from the cross-section of saidintermediate metal pipe in terms of shape.
 38. The cold roll formingmethod according to claim 37, wherein each of said diameter-reducingrolls of at least one of said plural sets of diameter-reducing rolls hason a surface thereof a pattern of ridges and grooves, such that passingsaid intermediate metal pipe through said plural sets ofdiameter-reducing rolls results in said pipe member having the patternof ridges and grooves formed on a surface thereof.
 39. The cold rollforming method according to claim 37, wherein disposed in a bore formedby said diameter-reducing rolls, of at least one of said plural sets ofdiameter-reducing rolls, is a molding die having on an outercircumferential surface thereof a pattern of ridges and grooves, withsaid molding die being inserted into said intermediate metal pipe uponpassing said intermediate metal pipe through said plural sets ofdiameter-reducing rolls such that said pipe member has on an innercircumferential surface thereof the pattern of ridges and grooves. 40.The cold roll forming method according to claim 37, wherein passing saidintermediate metal pipe through said plural sets of diameter-reducingrolls results in a diameter of said intermediate metal pipe beingreduced by at least 3%.
 41. A metal pipe product produced by the methodof claim
 37. 42. A cold roll forming method for reducing a diameter of araw metal pipe, comprising: passing a double pipe, including an originalouter metal pipe surrounding an inner metal pipe and having an originaldiameter, through plural sets of pre-forming rolls, thereby pre-formingsaid outer metal pipe into an intermediate outer metal pipe having oneof an elliptical cross-section, elongated circular cross-section,rectangular cross-section, and polygonal cross-section, with the provisothat the cross-section of said intermediate outer metal pipe isdifferent from the original cross-section in terms of shape; and thenpassing said double pipe through plural sets of diameter-reducing rollspositioned downstream said plural sets of pre-forming rolls, therebyforming said intermediate outer metal pipe into an outer pipe memberhaving (i) an outer circumferential length that is smaller than an outercircumferential length of said original outer metal pipe, and (ii) across-section that is different from the cross-section of saidintermediate outer metal pipe in terms of shape, with an innercircumferential surface of said outer pipe member being tightly engagedwith an outer circumferential surface of said inner metal pipe.
 43. Thecold roll forming method according to claim 42, further comprising:forming said double pipe by inserting said inner metal pipe into saidoriginal outer metal pipe such that said inner metal pipe and saidoriginal outer metal pipe overlap one another along a total length ofsaid original outer metal pipe.
 44. The cold roll forming methodaccording to claim 42, wherein each of said diameter-reducing rolls ofat least one of said plural sets of diameter-reducing rolls has on asurface thereof a pattern of ridges and grooves, such that passing saiddouble pipe through said plural sets of diameter-reducing rolls resultsin said outer pipe member having the pattern of ridges and groovesformed on a surface thereof.
 45. The cold roll forming method accordingto claim 42, wherein passing said double pipe through said plural setsof diameter-reducing rolls results in a diameter of said intermediateouter metal pipe being reduced by at least 3%.
 46. A metal pipe productproduced by the method of claim
 42. 47. The cold roll forming methodaccording to claim 37, wherein said raw material pipe has a wallthickness within a range of from 1.6 mm to 10.3 mm.
 48. The cold rollforming method according to claim 37, wherein said raw metal pipe has awall thickness and a diameter, with a ratio of said diameter to saidwall thickness being within a range of from 20.0 to 52.9, and passingsaid intermediate metal pipe through said plural sets ofdiameter-reducing rolls results in a diameter of said intermediate metalpipe being reduced by 3%-22%.
 49. The cold roll forming method accordingto claim 42, wherein said double pipe has a double wall thickness equalto a wall thickness of said original outer metal pipe plus a wallthickness of said inner metal pipe, with said double wall thicknessbeing within a range of from 1.6 mm to 10.3 mm.
 50. The cold rollforming method according to claim 42, wherein said double pipe has adouble wall thickness equal to a wall thickness of said original outermetal pipe plus a wall thickness of said inner metal pipe, and saidoriginal outer metal pipe has a diameter, with a ratio of said diameterto said double wall thickness being within a range of from 20.0 to 52.9,and passing said double pipe through said plural sets ofdiameter-reducing rolls results in a diameter of said intermediate outermetal pipe being reduced by 3%-22%.